1. Field of the Invention
The present invention relates to a color image forming apparatus with plural photoconductor drums for forming color images by modulating the power of plural lasers and exposing plural photoconductor drums.
2. Description of the Prior Art
As color laser beam printers and other color imaging technologies have become more common, demand for high resolution, excellent half-tone reproduction, and high speed processing and throughput has risen. Half-tone reproduction is achieved in these conventional color laser printers by pulse width modulation (PWM) of the laser beams.
The operation of a conventional color image forming apparatus is described below with reference to FIG. 12 showing a block diagram of a conventional laser modulation circuit, and FIG. 13 showing a timing chart of the laser modulation circuit in FIG. 12 using conventional pulse width modulation.
In this circuit, the digital/analog (D/A) converter 102 converts the raster scan digital image signal 101 to an analog image signal 103. The 1/2 frequency divider 104 1/2-frequency divides the pixel clock 119 of the digital image signal 101 to output the screen clock 120. The pattern signal generators A 105, B 106, and C 107 generate and output the corresponding pattern signals A 108, B 109, and C 110 based on the screen clock 120. The period of the pattern signals A 108, B 109, and C 110 is twice that of the pixel clock 119, and the waveform of each signal is different.
The comparators 111, 112, and 113 compare the analog image signal 103 with the pattern signals A 108, B 109, andC 110, respectively, to output the corresponding pulse width modulated PWM signals A 121, B 122, and C 123. The density gradient detection circuit 116 detects the density gradient in the main scanning direction of the digital image signal 101, and outputs the density gradient detection signal 117. The PWM signals A 121, B 122, and C 123 and density gradient detection signal 117 are input to the selector 114, which selects one of the PWM signals A 121, B 122, or C 123 based on the density gradient detection signal 117 to output the PWM signal 115.
Referring to FIG. 13, the first pattern signal A 108 is a ramp wave with a right up-ramp. The second pattern signal B 109 is a chopping wave. The third pattern signal C 110 is a ramp wave with a right down-ramp. The density gradient detection circuit 116 shown in FIG. 12 detects both the direction and the magnitude of the density gradient of the image signal to determine which of the PWM signals should be selected by the selector 114. The bottom row in FIG. 13 shows the PWM signals selected by the selector 114.
This conventional pulse width modulation circuit can prevent a loss of resolution in text output and avoid jagged edge lines appearing in line art even when image signals containing text and line art are pulse width modulated even though the period of the pattern signal is twice the pixel clock. (cf. Japanese patent laid-open number H2-47973)
When forming color images, however, it is necessary to record, at least, cyan, magenta, and yellow color images to the recording medium, and a screening process is performed to assign a different screen angle to each color to avoid differences in the relative positions of these colors on the recording medium from affecting intermediate tones. Also, by the screening processing, the ratio of the overlapping color dots per a unit area is controlled to produce less registration error for each color on the recording medium. Because the formed image is a screen of discrete lines, however, differences in the registration error of each color on the recording medium result in a variable ratio between overlapping color areas. This results in inaccurate intermediate color tones.
This problem is amplified in a color image forming apparatus using plural laser and plural photoconductors to form a full color image by overlaying plural single-color images each formed by one of the plural lasers because it is extremely difficult to adjust the relative position of each color on the recording medium so that color tone shifts are not a problem. This difficulty is due to differences in the length of the laser beam path with each color, errors in the parallelism of the laser scan lines, transfer and positioning accuracy of the recording medium, positioning errors in the beam detector installation, and other factors related to the mechanical precision of the apparatus.
While methods using pseudo-screening combining dithering and pulse width modulation within the range of a single pixel have been developed (U.S. Pat. No. 5,081,528), a certain loss of resolution is inevitable because of the use of dithering, which is basically one type of area gradation.